Regex Syntax Rules

Regular expressions follow precise syntax rules. Special characters (. * + ? ^ $ { } [ ] | ( ) \) have meaning and must be escaped with a backslash when you want them as literals. Parentheses must be balanced. Character classes must be closed. Quantifiers need a preceding element to repeat. This tool checks all these rules in real-time.

JavaScript vs Other Regex Engines

This validator uses JavaScript's RegExp engine, which supports: named groups (?<name>...), lookbehind (?<=...), the s (dotAll) flag, and Unicode property escapes. Features not supported: atomic groups, possessive quantifiers, conditional patterns, and recursive patterns (available in PCRE/PHP).

Tips for Writing Valid Patterns

Build patterns incrementally — start with a simple match and add complexity one piece at a time. Use non-capturing groups (?:...) when you don't need the capture. Be specific with character classes instead of relying on .*. Test against both matching and non-matching inputs to verify your pattern doesn't over-match.